Astron. Astrophys. 345, 448–460 (1999)
ASTRONOMY
AND
ASTROPHYSICS
ESO imaging survey
III. Multicolor data near the South Galactic Pole
I. Prandoni1,2, R. Wichmann1,3, L. da Costa1, C. Benoist1,4, R. M´endez1,5, M. Nonino1,6, L.F. Olsen1,7, A. Wicenec1, S. Zaggia1,8, E. Bertin1,9,10, E. Deul1,9, T. Erben1,11, M.D. Guarnieri1,12, I. Hook1, R. Hook13, M. Scodeggio1, and R. Slijkhuis1,9
1 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei M¨unchen, Germany 2 Istituto di Radioastronomia del CNR, Via Gobetti 101, I-40129 Bologna, Italy
3 IUCAA, Post Bag 4, Ganeshkhind, Pune 411007, India
4 DAEC, Observatoire de Paris-Meudon, 5 Pl. J. Janssen, F-92195 Meudon Cedex, France 5 Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
6 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-31144 Trieste, Italy 7 Astronomisk Observatorium, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark 8 Osservatorio Astronomico di Capodimonte, Via Moiariello 15, I-80131 Napoli, Italy 9 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
10 Institut d’Astrophysique de Paris, 98bis Bd Arago, F-75014 Paris, France
11 Max-Planck-Institut f¨ur Astrophysik, Postfach 1523, D-85748 Garching bei M¨unchen, Germany 12 Osservatorio Astronomico di Pino Torinese, Strada Osservatorio 20, I-10025 Torino, Italy
13 Space Telescope – European Coordinating Facility, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei M¨unchen, Germany Received 17 July 1998 / Accepted 16 December 1998
Abstract. This paper presents multicolor data obtained for a 1.7 square degree region near the South Galactic Pole (patch B) as part of the ESO Imaging Survey (EIS). So far the observa-tions have been conducted inB, V and I, but are expected to be complemented by observations in the U-band later in 1998. Object catalogs extracted from single exposure images are 80% complete down toB ∼ 24, V ∼ 23.5 and I ∼ 22.5, and once co-added should reach about 0.5 mag deeper. The data are be-ing made public in the form of catalogs, pixel maps, target lists and image “postage stamps”, which can be retrieved from the Web. Counts of stars and galaxies and the angular two-point correlation function of galaxies are computed and compared to other available data to evaluate the depth and uniformity of the extracted object catalogs. In addition, color distributions of stel-lar objects are presented and compared to model predictions to examine the reliability of the colors. The results suggest that the overall quality of the catalogs extracted from the images is good and suitable for the science goals of the survey.
Key words: catalogs – surveys – stars: statistics – galaxies: statistics
1. Introduction
The present paper is part of a series presenting the data accumu-lated by the public ESO Imaging Survey (EIS) being carried out in preparation for the first year of regular operation of the VLT.
Send offprint requests to: L. da Costa
As described in previous papers (Renzini & da Costa 1997, Non-ino et al. 1999, hereafter paper I) the main science goal of EIS is the search for rare objects such as clusters of galaxies, spanning a broad redshift range, quasars at intermediate and high redshifts, high-redshift galaxies and stars with special characteristics (e.g., white dwarfs, very low mass stars, brown dwarfs). These goals have guided the adopted survey strategy which, for EIS-wide, envisioned observations in V and I to search for clusters and in four passbands (U, B, V, I) over ∼ 1.7 square degrees in a region near the South Galactic Pole (EIS-wide patch B). The observations inB, V and I have already been completed, while observations in theU-band are expected to be carried out in the fall of 1999.
0 10 20 30 0 10 20 30 0.5 1 1.5 2 2.5 3 0 10 20 30
Fig. 1. Histogram of the seeing distribution for patch B obtained from
all observed frames and from the frames actually accepted for the sur-vey (shaded area). Vertical lines refer to 25, 50 and 75 percentiles of the accepted frames distribution. The three panels refer to the three observed bands (B, V, I), as indicated in each panel.
target lists. This is a key element for the exploration of the full range of science offered by the multicolor surveys envisioned for the new wide-field camera (WFI@2.2m) at the ESO/MPIA 2.2m telescope at La Silla.
In Sect. 2, the observations are described and the charac-teristics of the multicolor data are presented. This section also presents the extracted object catalogs and discusses their com-pleteness and reliability. In Sect. 3 the catalogs are evaluated by comparison with models and other data. Concluding remarks are presented in Sect. 4.
2. Observations and data reduction
2.1. Observations
The observations of patch B were carried out over several months in the period July 1997 to December 1997, using the red channel of the EMMI camera on the 3.5m New Technol-ogy Telescope (NTT) at La Silla. The red channel of EMMI is equipped with a Tektronix 2046× 2046 chip with a pixel size of 0.266 arcsec and a useful field-of-view of about90 × 8.50. EIS uses a special set ofBV I filters and the response function of the system can be found in paper I.
As described in paper I, the observations were carried out by a sequence of overlapping exposures (hereafter referred to as even/odd frames) 150 sec each, with each position on the sky be-ing sampled at least twice. A total of 701 frames were obtained in the area with 200 inB, 282 in V and 219 in I bands. Only 150 frames in each band were required to cover the field but poor weather conditions required several frames to be re-observed. The strong variations in the observing conditions can be seen in Fig. 1 which shows, for each band, the seeing distribution of all observed frames. For comparison the shaded histograms show the seeing distribution of the frames finally accepted, with the solid vertical line in each panel indicating the median seeing for each band. TheB-band is the worst overall with a median see-ing of 1.2 arcsec and with a few frames extendsee-ing to very large seeing (∼ 2.5 arcsec). The upper and lower quartiles are, as can
0 10 20 30 40 0 20 40 60 24 25 26 27 0 10 20 30 40
Fig. 2. Limiting isophote distributions from patch B frames actually
accepted for the survey. Vertical lines refer to 25, 50 and 75 percentiles of the distributions. The three panels refer to the three observed bands (B, V, I).
be seen in Fig. 1∼1.0 and ∼1.3 in B, ∼ 0.8 and ∼1.2 for V and ∼0.9 and ∼1.2 in I. In the analysis below 9 frames with seeing >
∼1.8 arcsec were discarded because of their incompleteness at faint magnitudes. Fig. 2 shows, again for each band, the1σ lim-iting isophote within 1 arcsec. The transparency of the nights also showed significant variations especially for theI-band im-ages, with one frame reaching 21 mag/arcsec2(not shown in the figure). For this frame, which was removed from the analysis, the depth reached is considerably shallower than the remain-ing frames leadremain-ing not only to a bright limitremain-ing magnitude but also to the detection of a significant number of spurious objects. Other frames with bright limiting isophotes have no significant impact in the analysis presented in Sect. 3.
Figs. 3 and 4 show, for each band, the two-dimensional dis-tribution of the seeing and limiting isophote as determined from the even frames. Similar results are obtained for the odd frames which alternate with the even ones. Such maps allow the poten-tial user of the derived catalogs to evaluate their reliability. The final data are reasonably homogeneous with the median seeing in all bands< 1.2 arcsec. However, some poor images do exist and for some applications must be removed, as discussed above. The area that is affected is<∼0.1 square degree.
Finally, it is worth mentioning that since the completion of paper I, theV images for patch A over an area ∼ 1.1 square degrees have also been reduced and are being made available together with the catalogs extracted from them which are used below.
2.2. Data reduction
450 I. Prandoni et al.: ESO imaging survey. III
Fig. 3. Two-dimensional distribution of the seeing as measured for
patch B for all the accepted even frames. Contours refer to 25, 50 and 75 percentiles of the distribution. The three panels refer to the three observed bands (B, V, I) from top to bottom.
especially in the preparation of object catalogs. Computation of colors require reliable association of objects detected in differ-ent passbands, which may be affected differdiffer-ently by the seeing, astrometric errors, the morphology of the objects and the per-formance of the de-blending algorithm. One also needs a proper definition for the measurement of colors for faint sources and upper limits for non-detections. Currently, preliminary color catalogs are being produced only for point-sources, which are being systematically inspected to verify the catalogs and iden-tify any peculiarities (Zaggia et al. 1999). This is an important first step towards the preparation of the final color catalog for patch B. For instance, during the visual inspection of objects selected by their peculiar colors, it became evident that ghost images, observed near relatively bright stars (V <∼14) in the B andV images, contaminate the catalogs.
Fig. 4. Two-dimensional distribution of the limiting isophote as defined
in the text estimated from the accepted even frames for patch B. Con-tours refer to 25, 50 and 75 percentiles of the distribution. The three panels refer to the three observed bands (B, V, I), from top to bottom.
2.3. Color transformation
Using all the standard star observations carried out in photo-metric nights at the NTT with the EIS filters, in the period July 1997-March 1998, the color transformation between the EIS and the Johnson-Cousins systems has been determined. In Fig. 5 the observed transformations for all the three bands are shown, as a function of color in the Johnson-Cousins system. The fits are given by the relations:
BEIS = B − 0.132(B − V ) (1)
VEIS = V + 0.047(B − V ) (2)
VEIS = V + 0.045(V − I) (3)
Fig. 5. Relation between the EIS and Johnson-Cousins system as a
function of color. Shown are all the standard stars observed under pho-tometric conditions in the period July 1997-March 1998.
Note that the transformation given here for IEIS is slightly different from that determined in paper I. This is because more standards have been included since and a more careful pruning of the data has been performed. The determination of color corrections include 284 measurements in B, 255 in V and 209 in I, with the formal errors in the color terms estimated to be <
∼ 0.02 mag in all three bands. In general, the color term is small except for the B-band. In this case the data also suggests a possible departure from linearity at the red end. As a final note, it is worth mentioning that in the process of examining all the standard star observations, errors in positions and the presence of variable stars in the Landolt lists were found. A complete list of these problems will be reported elsewhere.
2.4. Calibration
The photometric calibration of the patch was carried out by first bringing all frames to a common zero-point as determined from the relative magnitudes of objects in overlap regions, within a pre-selected magnitude range. This was done by a global least-square fit to all the relative zero-points, constraining their sum to be equal to zero. The internal accuracy of the derived pho-tometric solution is <∼0.005 mag (Paper I). Second, absolute
Fig. 6. Distribution of frames obtained at the 0.9m Dutch (D) telescopes
at La Silla overlapping the surveyed region of patch B. Also shown are parts of two DENIS strips that cross the field and the Lidman & Peterson fields (L) within the surveyed area. The hatched area represents regions containing EIS frames observed under photometric conditions.
zero-points are found for frames observed in photometric con-ditions. The zero-points for these frames were determined using a total of 36 frames of 7 fields containing standard stars taken from Landolt (1992 a,b), observed over 5 nights. These frames were also reduced through the pipeline, which identified the standard stars and measured magnitudes through Landolt aper-tures automatically (see paper I). Altogether 148 independent measurements of standards were used in the calibration.
Two solutions are then determined: one which computes a single zero-point offset, based on the weighted average of the zero-points of the calibrated frames, and the other using a first-order polynomial in both right ascension and declination. Comparison with external data suggests that a zero-point offset provides an adequate photometric calibration for the entire patch (see below).
452 I. Prandoni et al.: ESO imaging survey. III
Fig. 7. Projected distribution of stars
(left panel) and galaxies (right panel) detected in the passbands B (top
panels),V (middle panels) and I (bottom panels). The limiting magnitude
cor-responds to the star/galaxy classifica-tion limit for stars and to the estimated 80% completeness limit for galaxies (see text). Frames taken under extremely large seeing or with large extinction have been eliminated.
2.5. Object catalogs
During the processing of a patch through the pipeline, object catalogs extracted from single frames are merged together into a “patch” catalog for each passband. This is the parent catalog which consists of multiple entries of objects detected in over-lapping frames. For each detection, the seeing and noise of the frame in which the object was found are also stored. The parent catalog is used to derive different types of single-entry catalogs detected from 150 sec exposures such as the odd/even cata-logs described in paper I. Alternatively, it has also been used to derive a unique catalog (hereafter “best” catalog) defined by examining the characteristics of the frames where a given
ob-ject was detected, saving only the entry associated with the best seeing frame. Details regarding the methodology of association will be described elsewhere (Deul et al. 1999). From the flag information available in the single-entry catalog, filtered cata-logs have been produced for analysis purposes. The filtering is required in order to eliminate truncated objects and objects with a significant number of pixels affected by cosmics and/or other artifacts. The parameters adopted in the filtering are the same as those given in paper I.
been made to produce a color catalog combining the informa-tion of the catalogs derived from each passband. Using the same association scheme mentioned above, a cross-identification of objects is made and colors are computed using the mag auto es-timator of SExtractor (e.g., Paper I) which should be adequate for point sources. For non-detections in a given band, 1σ limiting magnitudes are computed from the seeing and noise properties of the best seeing frame available at the expected position of the object. Even though still rudimentary, this derived catalog serves for verification purposes and for a first cut analysis of the data. The final color catalog will only be derived from the co-added images. In this case, colors will be computed using the detection area determined from a reference image (one band, e.g.,I, or the summed images of different bands,e.g., V + I), but measuring the flux in the respective images. Even though the required software is available it is only now being integrated into the pipeline.
It is important to emphasize the complexity of handling and merging information extracted from different passbands. For instance, each object may have a different SExtractor stellarity-index which may impact the galaxy/star classification, close pairs may be de-blended in one passband and not in another, depending on the seeing. Clearly a complete description of all the possible pitfalls and the overall performance of the software is beyond the scope of the present paper, and will instead be discussed in Deul et al. (1999). The current work also shows the shortcomings of handling catalogs and points out the need for the implementation of an object database with a flexible user interface to allow for the full exploration of the data by different groups.
For the purposes of the present paper galaxies are objects with stellarity index< 0.75 if brighter than the star/galaxy clas-sification limit (B=22, V =22, I=21) or any object, regardless of the stellarity index, fainter than this limit. Stars are objects with stellarity index≥ 0.75. Note that this definition leads to some cross-contamination but it has no significant impact on the conclusions. In evaluating the data in Sect. 3 the derived star and galaxy catalogs were, for simplicity, trimmed at the edges and the bad frames discussed above were removed. After trim-ming the covered areas are: 1.3, 1.4 and 1.37 square degrees in B, V, I, respectively. The corresponding two-dimensional dis-tributions of stars, down to the star/galaxy classification limits, and galaxies, down to estimated 80% completeness limits (dis-cussed below), are shown in Fig. 7. The total number of objects in these plots are: 2290 stars brighter thanB = 22 and 33133 galaxies brighter thanB = 24; 3378 stars brighter than V = 22 and 58590 galaxies brighter thanV = 24; and 4297 stars and 44546 galaxies brighter thanI = 21 and I = 22.5, respectively. Recall that the distribution shown is for the “best” catalog. If one wishes to work with the odd/even catalogs their distribution have to be examined in the same way. In order to avoid extra-neous colors due to bad data in one or more bands, the color catalog examined in Sect. 3 corresponds to the common areas covered in the different passbands and has a total area of 1.27 square degrees.
2.6. Completeness, contamination and magnitude Errors
As in paper I, the completeness of the derived catalogs has been established by using a single field where several exposures have been made over the period of observations of patch B. The cor-responding catalogs in each passband were then compared with that of a typical frame with an exposure time of 150 sec. A total of 9 exposures in B band, 6 exposures in V band and 8 in I band, were selected discarding others taken in less favorable condi-tions. These exposures were coadded and the derived catalogs are at least one magnitude deeper than the typical survey frame. Comparing the identified objects in a single exposure frame with those extracted from the co-added images one can derive the expected completeness for typical 150 sec survey frames. The results of these comparisons are shown in Fig. 8 showing that the catalogs are 80% complete atB ∼ 24, V ∼ 23.5 and I ∼ 22.5. These limits should apply for survey frames with a seeing close to the median seeing. Also shown in the figure is the number of false positives computed from the fraction of objects identified in the single exposure frames which were not detected in the co-added image.
In order to estimate the accuracy of the magnitudes the odd and even catalogs were compared and a lower limit estimate of the photometric errors was obtained from the repeatability of the magnitudes for paired objects. The estimated errors from this comparison are given in Fig. 9, which shows that in the interval
16 < I < 20.5 they range from 0.02 to 0.1 mag, reaching 0.3
mag atI ∼ 23. Similar values are found for B and V brighter than∼ 24. These values correspond well to those estimated by SExtractor.
454 I. Prandoni et al.: ESO imaging survey. III
Fig. 8. Completeness (solid line) and expected contamination by
spuri-ous objects (dashed line) in the EIS catalogs for the different passbands considered. The computation of these quantities are described in the text.
In order to further investigate possible systematic errors in the photometric zero-point over the scale of the patch, the EIS catalogs were also compared with object catalogs extracted from the two DENIS strips that cross the survey region (see Fig. 6). This allows one to investigate the variation of the zero-point as a function of right ascension and, especially, of declination. The results are shown in Fig. 11. The domain in which the com-parison can be made is relatively small because of saturation of objects in EIS at the bright end (I ∼ 16) and the shallow magni-tude limit of DENIS (I ∼ 18). Still, within the two magnitudes where comparison is possible one finds a roughly constant zero-point offset of less than 0.02 mag for both strips and a scatter that can be attributed to the errors in the DENIS magnitudes (Deul 1998).
Fig. 9. Comparison between the estimated error in the magnitudes from
the odd/even comparison (solid squares) and the SExtractor estimates (open squares).
16 18 20 -1 -0.5 0 0.5 1 I 16 18 20 -1 -0.5 0 0.5 1 I
Fig. 10. Comparison of the EIS data inI-band
with Lidman & Peterson (1996) catalog for the two fields in common. Also shown are the mean and the rms in 0.5 mag bins.
15 16 17 18 -1 -0.5 0 0.5 1 I 15 16 17 18 -1 -0.5 0 0.5 1 I
Fig. 11. Comparison of the EISI-band
magni-tudes with those measured by DENIS for the two strips that overlap patch B. Also shown are the mean and the rms in 0.5 mag bins.
Fig. 12. Comparison of the EIS magnitudes inB, V and I (top to
bottom) with those measured from observations of the Dutch 0.9m telescope. Objects in the three fields available have been combined.
significant gradients in the photometric zero-point in any of the passbands.
In summary, comparison of the EIS magnitudes with avail-able external data shows no indication of gradients in the
pho-tometric zero-point of the patch. However, the external data are mainly overlapping the EIS data at low declination (δ<∼−29.5), especially for theB− and V −band, giving weaker constraints in that direction for these bands. Instead, the consistency of the zeropoints for these bands were investigated using the(V − I) for halo stars in the turn-off region around(V − I) ' 0.7. It was found that the(V −I) of these stars showed a gradient with declination. To localize the problem the(B − V ) and (B − I) were also checked for gradients in the direction of declination and the former was found to have gradient. Therefore, a linear relation (Eq. 5) was fitted to remove this gradient.
Vcorr= Vold+ 0.24 ∗ (30 + dec) (5)
This correction has been applied to the catalogs and a second version of the data will be released in 1999, to take this correc-tion into account.
3. Data evaluation
456 I. Prandoni et al.: ESO imaging survey. III
Fig. 13. The EIS differential star counts versus magnitude compared
to the galactic model predictions (solid line), as described in the text. The model includes an old disk population (dashed line), a thick disk component (long-dashed line) and a halo (dotted line).
3.1. Point-like sources
Fig. 13, shows the comparison of the star counts for patch B de-rived using the stellar sample extracted from the object catalogs produced in each passband (Sect. 2.5), with the predicted counts based on a galactic model composed of an old-disk, a thick disk and a halo. The star- and color-counts presented in this section have been computed using the model described by M´endez & van Altena (1996), using the standard parameters described in their Table 1. It is important to emphasize that no attempt has been made to fit any of the model parameters to the observed counts. The model is used solely as a guide to evaluate the data and, as can be seen from Fig. 13 one finds a remarkable agree-ment with the predicted counts down to the magnitude where the star-galaxy separation is expected to become unreliable. The excess in counts seen at the bright end is due to the saturation of brighter objects (V <∼16).
Using the preliminary color catalog for point sources the observed color distribution of stars brighter thanV = 21 is compared to model predictions in Fig. 14 over three ranges of magnitude as indicated in each panel. The model computes
star-counts inB, V and I by adopting a series of color-magnitude diagrams appropriate for the disk, thick-disk and halo of our Galaxy. In order to output predicted counts in the natural pass-bands of the EIS survey, the transformation given by Eqs. (1)-(4) have been used to convert from the EIS magnitudes to the Johnson-Cousins system in such a way that the predicted counts are actually evaluated in the EIS passbands and are convolved using the error model given in Fig. 9. As can be seen from Fig. 5, the number of red ((B − V ) > 1.2) standard stars defining the color transformation is very small, so that one might ex-pect possible discrepancies between the observed and predicted counts, particularly for redder colors. One way of overcoming this would be to use synthetic star colors using the system re-sponse functions given in paper I. However, for the purposes of describing the usefulness of the data, the current calibration is sufficient. Considering that none of the model parameters have been adjusted to fit the present data, the good agreement of the model to the observed counts in both (B − V ) and (V − I) is remarkable, although some discrepancies can also be readily seen.
At the brightest magnitude bin one sees a deficit of red ob-jects relative to the model predictions in both (B − V ) and
(V − I), especially in the former, which is due to saturation
effects. Note that objects with saturated pixels have been dis-carded from the color catalog.
In the range18 < V < 20, the color-counts are known to split into two major peaks, each sampling a different stellar population (Bahcall 1986). The blue peak at(B − V ) ∼ 0.5 is due to halo stars near the turnoff (MV ∼ +4), located at few kpc from the Galactic plane. The red peak at(B − V ) ∼ 1.3 is due to faint M-dwarf stars from the disk, located at less than 1 kpc from the Sun. Note the small relative offset (∼ 0.1 mag) between the observed and predicted location of the red peak. As pointed out above this may be due to, and is consistent with, the departure of the color term from the linear correction adopted for objects with(B − V ) > 1.2. Note that the agreement is much better in(V − I) for which the contribution from color terms are expected to be negligible.
Traditionally, the observed splitting in the color peaks has been used to determine the local normalization of halo stars in the solar neighborhood. Note in this context the difference in the amplitude of the counts in the blue peak in the magnitude range 20 < V < 21, which is seen in both (B − V ) and
(V − I). Most photometric surveys at faint magnitudes (e.g.,
Reid & Majewski 1993) have relied on pencil-beam surveys covering a small fraction of a degree. Therefore, the number of observed objects per bin has been very small, leading to large uncertainties in the derived model parameters. The EIS sample, covering∼ 1.3 square degrees, represents a significant improvement and may allow for a better determination of these parameters.
Finally, it is interesting to point out the existence of a popu-lation of blue objects, in particular, the suggestion of a peak at
(B − V ) ∼ 0.15 observed at faint magnitudes (20< V < 21).
Fig. 14. Color distribution for point-sources
for different magnitude intervals as indi-cated in each panel. Also shown are predic-tions for the galactic model of M´endez & van Altena (1996).
the model. Instead the observed blue objects could consist of a mix of white dwarfs, blue horizontal branch stars or perhaps halo field blue stragglers. Further investigation on the nature of these objects seems worthwhile.
The results demonstrate that the stellar color catalog being produced is by and large consistent with model predictions and the observed differences may possibly point to deficiencies in the model which should be further investigated by interested groups. Although primarily driven by other goals, the above discussion shows that the EIS data is also useful for galatic studies.
3.2. Galaxies
In order to evaluate the quality and the depth of the galaxy samples, galaxy counts in the different passbands are shown in Fig. 15 and compared to those determined for patch A and by other authors as indicated in the figure caption. In these com-parisons theI magnitudes of Lidman & Peterson (1996) have
458 I. Prandoni et al.: ESO imaging survey. III
Fig. 15. EIS patch B galaxy counts (filled squares) in different
pass-bands (B, V, I, from top to bottom) compared to the counts obtained by: Lidman & Peterson (1996, triangles), Postman et al. (1996, dia-monds) and Arnouts et al. (1997) (open squares), as provided by the authors. Also shown are the counts obtained in patch A (stars) for the
V and I-band. The counts from other authors have been converted to
the Johnson-Cousins system, as described in the text.
Fig. 16 showsw(θ) obtained for each of the three passbands B, V, and I, using the estimator proposed by Landy & Szalay (1993). The calculation has been done over the area defined above (see Fig. 7). The error bars are1σ errors calculated from ten bootstrap realizations. The angular correlation function is, in general, well described by a power lawθ−γfor angular scales extending out toθ ∼ 0.5 degrees, with a value of γ in the range 0.7–0.8. The absence of any strong feature at the scale of the individual survey frame should be noted. Furthermore, no sig-nificant variations of the slope are detected, except at the bright end in all the three passbands. In this casew(θ) is somewhat flatter than at fainter magnitudes. A possible explanation is the presence of the nearby cluster (ACO S84) atz ∼ 0.1 located near the center of the patch. To test this hypothesis the correla-tion funccorrela-tion was recomputed by discarding a square region of about 0.2 degrees on the side centered at the nominal position of the cluster. Using the pruned sample yields a steeper corre-lation function for the patch, consistent with the expected slope
0 1 2 3 -3 -2 -1 0 1 -3 -2 -1 0 1 -3 -2 -1 0 1
Fig. 16. The angular two-point correlation function calculated for patch
B, in theB, V, and I bands as indicated. In each panel the three curves correspond (from top to bottom) to the limiting magnitudes 21, 22, 24 (inB); 20, 22, 24 (in V ); 19, 21, 23 (in I). The dotted line represents a power law with a slope of−0.8. The error bars are 1σ errors obtained from 10 bootstrap realizations.
of 0.8. These results show the uniformity of the EIS catalogs, once obviously bad frames, selected on the basis of seeing and limiting isophote, are discarded.
Fig. 17. Amplitude of the correlation function measured at 1 arcmin for
patch A (open squares), and patch B (full circles) and after removing the nearby S84 cluster (full squares). These results are compared to those from Roche et al. (1993) (big open triangles) and Jones et al. (1987) (small open triangles) in the B band, from Woods & Fahlman (1997) in the V band (open pentagon), and from Postman et al. (1998) in the I band (open circles).
from the EIS galaxy catalogs are consistent with those obtained by various authors over the entire range of magnitude. Note that forI>∼22 the EIS points lie slightly below those recently computed by Postman et al. (1998).
In summary, the above results are further evidence that the EIS galaxy catalogs are uniform within a patch, that the zero-points for the different patches are consistent and are in good agreement with external data.
4. Summary
In this paper multicolor data for patch B covering about 1.7 square degrees near the SGP have been presented. The quality of single band and color catalogs have been assessed by com-parisons of basic statistics such as number counts, color distri-bution, and angular correlation function, with model predictions and results from other authors. The results indicate that the EIS object catalogs are suitable for the science goals of the survey leading to consistent results when compared to other data. Fur-thermore, one finds good agreement between the results derived from the catalogs extracted from patches A and B.
The work being carried out is essential in the preparation of the final release of the EIS data. While the production of
single-frame catalogs is straightforward, the preparation of a catalog covering the whole patch requires some experimenta-tion in order to fine-tune the parameters and verify its unifor-mity, completeness, and reliability. Multicolor data adds to the complexity of the task of catalog production and further work in the preparation of well understood color catalogs is required. The experience gained so far points out the need for a sus-tained effort in the development of techniques and tools which will allow for: 1) the production of more customized catalogs for different science goals, essential for public surveys; 2) the exploration of the dimensional space offered by multi-color data; 3) the cross-correlation of the detected objects with the increasing number of databases available in different wave-lengths. Searches in this multi-dimensional space offer unique science opportunities and the implementation of suitable tools for its exploration represent a major challenge for the efficient use of imaging surveys carried out with the specific purpose of producing targets to feed 8-m class telescopes.
The full range of products for patch A and B in the form of astrometric and photometric calibrated pixel maps, object cata-logs, candidate target lists (Zaggia et al., 1999) and on-line co-added section images can be found at “http://www.eso.org/eis”. New products will be added incrementally as they become ava-ialble. Further information on the project are available on the World Wide Web at “http://www.eso.org/eis”.
Acknowledgements. We thank all the people directly or indirectly
in-volved in the ESO Imaging Survey effort. In particular, all the mem-bers of the EIS Working Group for the innumerable suggestions and constructive criticisms, the ESO Archive Group and ST-ECF for their support. We also thank C. Lidman for providing his images, zero-points and catalogs. We are also grateful to the DENIS consortium for making available some of their survey data. The DENIS project devel-opment was made possible thanks to the contributions of a number of researchers, engineers and technicians in various institutes. The DENIS project is supported by the SCIENCE and Human Capital and Mobil-ity plans of the European Commission under the grants CT920791 and CT940627, by the French Institut National des Sciences de l’Univers, the Education Ministry and the Centre National de la Recherche Sci-entifique, in Germany by the State of Baden-Wurttemberg, in Spain by the DGICYT, in Italy by the Consiglio Nazionale delle Richerche, by the Austrian Fonds zur F¨orderung der wissenschaftlichen Forschung und Bundesministerium f¨ur Wissenschaft und Forschung, in Brazil by the Fundation for the development of Scientific Research of the State of S˜ao Paulo (FAPESP), and by the Hungarian OTKA grants F-4239 and F-013990 and the ESO C & EE grant A-04–046. Our special thanks to A. Renzini, VLT Programme Scientist, for his scientific input, support and dedication in making this project a success. Finally, we would like to thank ESO’s Director General Riccardo Giacconi for making this effort possible.
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